lol thanks

Hoo-Boy, wish I had just ONE customer recite auto history as completely. I'd usually have to imagine what their washing machine sounded like on spin, then apply that to the car.
I have to agree with CPOM, dealerships need revenue due to high overhead. Very few have good repair personel, dispite having superior diagnostic equipment and literature.
I am also in agreement with the intake gasket being the main culprit, and setting the codes. Even the SMALLEST vacuum hose leaking will eventually send the O2 sensor to the rubber room. Figure open throttle, max'd TPS signal, O2 sensor's brows raised, all hands on deck in the ECM altering timing and monotoring engine temp and RPM etc. Then along comes extra air from the manifold leak, the ECM has already adjusted for max everything so can only fail what it can't compute.
I'd suggest changing the intake gaskets, it's the cheapest item on your list, and will probably bring the best result. If not, post again. There's a lot of bright minds on this forum willing to help...Cal

Ok, just got back from school. Checked AllData and the P1171 code that I had that showed up as "Unknown Powertrain DTC (Manufacturer Controlled; Fuel and Air Metering)" on my cheapy scan tool (CarChip e/x) is "Fuel system lean during acceleration" according to AllData.

Also talked to one of my teachers and told him the short version of the problem and he thinks the wire harness stuff is BS.

So I think that the drivability problem (what I can notice) is a partially plugged catalytic converter because replacing one fixed the drivability problem last time. But I think something not so noticeable is causing the catalytic converter to plug up.

Is it possible that with a cracked intake manifold gasket the ecm would be trying to make things richer so I would always be running rich and all that extra fuel could burn up in the catalytic converter causing it to plug up? For a while now I also had a hole in my fuel pressure regulator vacuum line and I think a slightly leaky fuel pressure regulator. So could a cracked intake manifold gasket, a hole in the pressure regulator vacuum hose, and a leaky fuel pressure regulator all on a car driven by someone with a lead foot (if that affects any thing) cause the catalytic converter to burn up/plug up?

Yes. You will run so rich from these leaks you can smell fuel at your exhaust pipes. Combustion burns colder due to excess fuel too, so you get more byproducts and particulates.

Still interested in what every one has to say about this.

But a little off topic, Which is a better IMG, OE or say FelPro? Are the new OE ones metal reinforced?

Once again, CPOM is on the money. Low vacuum to the fuel regulator=higher fuel pressure, your computer won't know about the vacuum leak, only that it has switched on the vac switching valve to the regulator, lowering fuel pressure. The pulse width at the injectors is set for LOWER pressure WITH vac, so you're dumping fuel, contaminating cats and freaking the O2 sensor.
It's quite possible the first jammed cat burst the intake gasket with hot backpressure. Auto computers can merely tell what is out of the norm, not why or what to replace. It's up to a good tech to analyze codes and determine a significant order of importance. Cheap scanners will show codes, but not values sent by senders. Half a dozen codes pumped out of an exasperated ECM might only really be a vac leak or weak temp sender.
I too, think the harness is BS. Your truck is 10 years newer than mine and surely has much less miles. My wiring is in perfect condition save for a few layers of off-road enjoyment. Perhaps there has been some chafing on a wire or two but certainly easier to repair than replace the whole dang thing! Try moving the harness yourself.
Change the gaskets and post the results for all to know, just 'cause it hasn't happened to us yet doesn't mean it's not going to...Cal

Ok, well I was planning on picking up the car tomorrow cuz that's the last time I'll be able to get a ride to pick it up for three weeks and order up a new OE pressure regulator and either OE or FelPro intake manifold gaskets, not sure which yet or if I should replace the common chamber and throttle body gaskets too, what do you guys think? And replace the gaskets, pressure regulator and regulator vacuum hose well my dad is out of town then bring it back to the dealer for a new cat or two. when he gets back, But since you say "It's quite possible the first jammed cat burst the intake gasket with hot backpressure" I'm worried I'll burst my new intake gasket on the way back to the dealer, But I'd hope the metal reinforced FlePro gasket would let that happen.

that won't happen don't worry about it and just get it done

Felpro or OE, both fine. Replace ALL associated gaskets. New and freshly torqued gaskets will stand up to the drive, try to keep yer foot light on the throttle...Cal

Sorry didn't have a chance to read this earlyer.

If you could wait untill Tuesday i will give you a deffinate answer to your problem, but from what you have posted so far it seems like CPOM is right (to some degree).
A bad IM gasket would cause you to run lean, but the simptoms & the codes you are have point more towards a bad MAF sensor.

You wouldn't beleave how many bad MAF's Mercedes has.
And not only them, i've replace more than 20 MAF's in the past 2 months on everything from ML320 to Maxima.

Do you have a scanner that can display live data? If no than do you know a mechanic or someone else who does?

You need to read the data displayed my MAF to see if it's within the acceptable range.
I'm not sayint it's the MAF deffinatley, but it's something that needs to be checked out.

Also get the freeze frame records to fully understand the coditions under which each of the codes are set.